令 `x=\tan(A/2)`, `y=\tan(B/2)`, `z=\tan(C/2)`,则 `x`, `y`, `z>0`, `xy+yz+zx=1`,有
\begin{align*}
\sin A&=\frac{2x}{1+x^2}=\frac{2x\sqrt{xy+yz+zx}}{(x+y)(x+z)},\\
\sin\frac A2&=\frac x{\sqrt{1+x^2}}=\frac x{\sqrt{(x+y)(x+z)}},
\end{align*}于是原不等式等价于
\[\sum\frac{4xy(xy+yz+zx)}{(x+y)^2(y+z)(z+x)}\leqslant3\sum\frac{xy}{(x+y)\sqrt{(z+x)(z+y)}},\]即
\[4\sum\frac{xy(xy+yz+zx)}{x+y}\leqslant3\sum xy\sqrt{(z+x)(z+y)},\] 因为
\begin{align*}
\LHS&=4\sum\left( xyz+\frac{x^2y^2}{x+y} \right)=12xyz+4\sum\frac{x^2y^2}{x+y},\\
\RHS&\geqslant3\sum xy\bigl(z+\sqrt{xy}\bigr)\geqslant3\sum xy\left( z+\frac{2xy}{x+y} \right)=9xyz+6\sum\frac{x^2y^2}{x+y},
\end{align*}故只需证
\[2\sum\frac{x^2y^2}{x+y}\geqslant3xyz,\]由柯西有
\[2\sum\frac{x^2y^2}{x+y}\geqslant\frac{(xy+yz+zx)^2}{x+y+z}\geqslant3xyz,\]所以原不等式获证。 |